A technique has been disclosed, wherein a center of two white lines on a road as road markings to indicate lane boundaries is used as a fixation point, a yaw rate, which is required to allow a vehicle to arrive at the fixation point, is calculated, it is determined that the vehicle may possibly depart or deviate from the lane if a difference between the required yaw rate and an actual or real yaw rate exceeds a threshold value, and thus the vehicle is subjected to the braking (vehicle is decelerated) (see, for example, Patent Document 1). According to the technique described in Patent Document 1, the lane departure can be precisely detected, and the vehicle can be decelerated when the lane departure is caused.